Before the 4E announcement my strongest dislike in D&D was against how races were handled. They all felt bland and pointless, like humans from different cultures rather than distinct and different. People roleplayed stereotypes, because if you didn't do the stereotype there was no way to know which race your character was.
I started writing my own campaign world then, throwing out all the usual PH races as they didn't add anything to the game. Instead added in races that were shapeshifters, insectoids, undead and faeries.
Before I got very far with the new world, the new edition was announced. And what I loved the most from the early promises was how they said they wanted race to matter more. Not everything has turned out the way I had hoped. (Need lots more racial feats and paragon paths!) Still I can't really complain when the improvement is this huge. Tieflings and Dragonborn might not fit your old setting - but they are exotic and distinct from the other races. And by making the races more distinct from scratch, it becomes easier to make individuals out of them, rather than fall into racial stereotypes.
That is why I love Dragonborn and Tieflings. And the rest of the PH races after the overhaul, except the Half-Elves which still seem random and bland. Well I love Tieflings even more, because I found a way to use them in my campaign that makes me smile every time I think of them.
Oh and Eladrin is perhaps my favourite race these days. I don't think of them as elves though. I think of them as fey, the race that has been most clearly missing from every old version of D&D (with the possible exception of 1E Oriental Adventures Fey Folk

).